Spark or atomic emission spectrometry, also known as optical emission spectroscopy, is a well-known technique for analysis of samples. A solid sample is subjected to a spark or electrical discharge which vaporises a portion of the sample and promotes atoms and ions into excited states. Light is emitted during relaxation of the excited atoms and ions into lower energy states, with subsequent spectroscopic analysis of the discharged light providing information on the material composition of the sample.
Spark or arc atomic emission spectrometers (AES) commonly comprise a spark stand (or spark table) having a sample holder consisting of a receiver plate or analysis table for mounting the sample. The receiver plate can be grounded or held at a fixed voltage and contains an aperture to a gas chamber below. The sample, which is larger than the aperture, can be mounted over the aperture to form a gas-tight seal between the sample and the receiver plate. Preferably, a clamp is used to hold the sample firmly in place over the aperture and secure the sample to the receiver plate. An electrode of the AES is arranged in the gas chamber proximate to but spaced apart from the aperture of the plate. Applying a voltage to the electrode ignites a spark or arc between the electrode and plate or sample. This causes a portion of the sample which is exposed to the spark through the aperture of the plate to be vaporised or ablated.
To avoid adulteration of the optical spectra, the sample is vaporised and excited in the presence of an inert gas such as argon. Typically, a flow of the inert gas is passed through the gas chamber which houses the electrode and in which the spark occurs. When the sample is correctly mounted on the receiver plate so as to form a gas-tight seal, air cannot enter the gas chamber or argon gas leak from the gas chamber. However, if the sample is mounted incorrectly, even small amounts of air leaking into the gas chamber can lead to anomalous spectra and so inaccurate analysis of the sample.
When the sample is correctly arranged on the receiver plate, the clamp typically provides a downward pressure to form the gas-tight seal between the sample and the receiver plate. However, if the sample is not correctly seated, the clamp can connect to the sample at an angle. This risks a poor seal being formed between the sample and the receiver plate. Moreover, the sample can sometimes be displaced during set-up or use of the AES, for example due to contact with the operator or because the sample is poorly secured to the sample holder and moves due to pressure exerted from the gas chamber below. This can cause the seal between the plate and sample to leak, thereby disrupting the sample analysis and potentially exposing the electrode to the operator.
Spark generators for generating the spark or arc provide a spark having a specific waveform profile for providing superior vaporisation and excitation of a sample. To generate the spark or arc, spark generators often use a combination of a high-current-controlled source and a high-voltage power supply. Each spark is a combination of a high-voltage discharge between the electrode and the sample and amplitude modulated current arranged to flow through the electrode, to the sample and the sample holder. When the sample is incorrectly located on the receiver plate, the spark can fail to ignite, despite a high voltage being present at the electrode. Not only can this causes errors or anomalies in any sample analysis, but also results in an inherent risk to the user due the hazardously high voltage levels created by the apparatus. Even if the spark is ignited, improper location of the device or faults within the device can result in serious electrical shocks or electrical burns to the operator.
DE 10 2005 057 919 provides an approach to trigger a safety shutdown of the AES in the event that the charge flowing through the electrode exceeds a threshold value during certain periods of the spark. In particular, the apparatus provides a number of arrangements of the circuitry of the spark generator to prevent an electric shock to the operator sufficient to be harmful to the human body. In one example, a small test voltage is applied between the electrode and the receiving area before initiating a spark. In this arrangement, if a voltage drop is observed between the electrode and the receiving area it is assumed that there is an unsafe connection, for example due to contact with the operator. As a consequence, the high-voltage power supply for energising the electrode is prevented from activation. Providing no current is passed between the electrode and the receiving area as a result of the test voltage, the apparatus is permitted to generate a spark.
Against this background, it is desirable to provide an apparatus and method that offers an improved arrangement for establishing safe operation of an atomic emission spectrometer (AES).